Zeolites are porous crystalline aluminosilicates which have found important applications as catalysts, adsorbents and ion exchangers. Many of these zeolitic materials have well-defined structures which form channels and cavities in their interior of uniform size and shape which permit the absorption of certain molecules, whilst they permit the passage to the inside of the crystal of other molecules of a too large size to be diffused through the pores. This characteristic gives these materials molecular screen properties. These molecular screens may include Si and other elements of the IIIA group of the periodic table, all tetrahedrically coordinated, the tetrahedrons being bound by their vertices through oxygens forming a three-dimensional network. The negative charge generated by the elements of the IIIA group in network positions is compensated by the presence in the crystal of cations, such as for example, alkalines or earth alkalines. A type of cation may be totally or partially exchanged by another type of cation by ion exchange techniques, thus being able to vary the properties of a given silicate by selecting the desired cations.
Many zeolites have been synthesized in the presence of an organic molecule which acts as structure-directing agent. The organic molecules which act as structure-directing agents (SDA) generally contain nitrogen in their composition, and give rise to stable organic cations in the reaction medium.
The mobilization of the silica can be carried out in the presence of OH− groups and basic medium, which can be included as hydroxide as the same SDA, such as, for example, tetrapropylammonium hydroxide in the case of the ZSM-5 zeolite. The fluoride ions can also act as mobilizing agents of silica in zeolite synthesis, as disclosed, for example, in patent EP-A-0337479 the use of HF in H2O at low pH as mobilizing agent of silica for ZSM-5 synthesis.